An investigation using fluorescent probes to study membrane structure and function is being carried out. The concentration dependence in egg lecithin vesicles of the absorbance, fluorescent intensity, lifetime and emissions polarization has been determined using the probes; N-phenyl-1-naphthylamine (NPN), 1-anilino-8-naphthylamine sulfonate (ANS), 12-(9-anthroyloxy) sterate (12AS), 1,6 diphenyl-1,3,5-hexatriene (DPH) and beta paranaric acid. We have also studied the emission wavelength dependent lifetime of NPN an the anthroyloxy probes in various solvents and egg lecithin bilayers. Evidence for lipid domains in membranes has been obtained by monitoring the effect of free fatty acids on ANS and DPH polarization as well as DPH lifetime heterogeneity. Studies have been carried out on vesicles prepared from human red cells in which the anion transport protein band 3 is enriched to 80-90%. These vesicles are found to be oriented inside out with respect to the intact red cell and only about 5-15% are sealed to small molecules. Non radiative energy transfer has been determined between the protein's tryptophan and the lipophilic anthroyloxy probes. Analysis of these results yields information on the spatial distributions of the tryptophan. Preliminary results indicate considerable conformation change with temperature and treatment with inhibitors of anion transport.